The homogeneous operating mode, preferably used for low and medium engine loads, consists in injecting fuel into the combustion chamber very early (for example during the engine intake phase) so as to obtain homogeneous mixing of the fuel with a fluid such as air or a mixture of air and recirculated exhaust gas (EGR).
For the conventional combustion operating mode, preferably used at high engine loads, a fuel injection is performed around the piston compression top dead center and a conventional combustion by self-ignition, then by diffusion occurs.
With this combustion mode, it is also possible to carry out an early fuel injection referred to as “pilot” injection, at the intake phase start for example.
In the homogeneous combustion operating mode, it is advantageous for the fuel injection to occur very early in the engine running cycle in order to obtain a homogeneous mixture, but there are risks of cylinder wall wetting by the fuel injected.
Thus, a fuel injection at the start of the intake phase has the advantage of confining the fuel injected in the bowl the piston of such an engine is usually equipped with, while limiting contact of this fuel with the cylinder wall, but the temperature of the fluid contained in the combustion chamber is not high enough. It is therefore difficult for the fuel injected in this bowl, then in the combustion chamber, to vaporize.
In the conventional operating mode, the pilot injection involves the same drawbacks as regards the injected fuel vaporization difficulties as those mentioned above.
This fuel vaporization difficulty can lead to disruptions in the progress of the fuel mixture combustion and to an increase in the discharge of pollutants into the atmosphere, as well as a fuel overconsumption by the engine.
For gasoline type indirect-injection engines, it is already known to perform vaporization by injecting this fuel in liquid form or in form of fine droplets into the exhaust gas contained in the intake pipe. Upon contact with this hot gas, the fuel vaporizes to mist and mixes not only with this gas, but also with the fluid fed thereafter into the combustion chamber of the engine.
Such a transposition to direct-injection engines is impossible, essentially because fuel injection cannot take place in the intake pipe.
However, better fuel vaporization essentially has the advantage of reducing carbon oxides (CO) and unburnt hydrocarbons (HC) emissions upon combustion of the fuel mixture.
The present invention aims to overcome the aforementioned drawbacks by means of a method using the components usually present in a direct-injection engine.